Pipeline components, such as valves, pumps, meters and the like are connected into the pipeline by means of pipe flanges, with gasket ring seals interposed between flange faces, where it is contemplated that the component may have to be separated for maintenance work, repairs or replacement. Weld neck flanges, which are commonly provided for such purposes are produced by the use of forging presses and suitable forging dies. These dies are rather expensive since they must be very heavy to contain the large forces which the hot steel forging exerts on the die, and the steels used in the dies are costly due to the alloy requirements needed to sustain the thermal shock involved in the operation.
Welding neck flanges, as produced in quantity for the piping trade, have tapered necks forged as a projection from the side of the flange opposite to the sealing gasket. This tapered neck has its outside diameter reduced at the point of welding to the pipeline to match the outside diameter of standard pipe sizes. Typically this would produce a flange neck wall thickness of 1/4" to 1/2" for the pipe sizes from 6" to 16" diameter. However, in valves and other pipeline pressure vessels, it is common to have body wall thicknesses of from 3/4" to 11/2" for those same pipe sizes. When such pressure vessels have their bodies made as steel castings, they usually have integrally cast pipe flanges with suitably heavy flange necks. However, where such pressure vessels are manufactured from wrought steel components, they may have end closures of thick steel plate with heavy steel cylindrical hubs welded around flow openings therethrough. Weld neck flanges which are commonly available may not be suitable for welding to such closure hubs because of their thin wall tapered necks above described. However, the alternative of providing forged pipe flanges that have suitably heavy necks may not be economically feasible because the cost of special forging dies is very high, and the setup charges for the forging press are considerable. This can be a major manufacturing consideration, particularly with the great variety of flanges required, and such costs are usually unwarranted just to satisfy the relatively small demand for special flanges, as compared to the large market for standard flanges which are produced with tapered necks for welding to pipe.
Added to the cost burden of forged special flanges with heavy necks is the delivery delay which is usually encountered. Since the run is generally short, the forge shop will not find the job attractive and the delivery time obtainable for the forgings is normally not suited to the usual fabrication schedules for valves or similar products.